Calcium, 24-Hour Urine

Calcium, 24-Hour Urine (urine calcium, quantitative; urinary Ca2+) is used to appraise calcium balance across the gastrointestinal tract, skeleton, and kidneys. It assists in assessing dietary intake and intestinal absorption, investigating increased skeletal calcium mobilization such as in osteoporosis, and identifying renal disorders that alter tubular calcium reabsorption or excretion. The test also supports evaluation of parathyroid gland function and aids in distinguishing familial hypocalciuric hypercalcemia from other causes of altered calcium homeostasis.

A portion of filtered calcium is eliminated in urine each day, and this assay measures that daily renal calcium loss. Calcium absorbed from the gastrointestinal tract circulates in blood and contributes to bone mineralization, neuromuscular transmission, muscle contraction, and coagulation. The kidneys filter calcium and normally reabsorb most of it; when serum calcium increases, tubular reabsorption falls and urinary calcium excretion rises. Calcium and phosphate concentrations typically move inversely, with higher serum calcium often accompanying lower phosphate and vice versa. Endocrine control involves parathyroid hormone, released in response to hypocalcemia or hyperphosphatemia, which stimulates osteoclastic bone resorption and increases circulating calcium. Calcitonin counters hypercalcemia by promoting movement of calcium from blood into bone. Parathyroid hormone also drives renal activation of vitamin D, which enhances intestinal calcium uptake and supports renal tubular reabsorption. By capturing total urinary calcium over 24 hours, the test helps assess calcium intake and the balance among gastrointestinal absorption, bone resorption, and renal excretion.